In recent years, in the field of radio communication, especially in mobile communication, a variety of information such as images and data in addition to speech is transmitted. The demand for higher-speed transmission is expected to further increase in the future, and, to perform high-speed transmission, a radio transmission techniques that utilizes limited frequency resources more effectively and achieves high transmission efficiency is in demand.
OFDM (Orthogonal Frequency Division Multiplexing) is one of radio transmission techniques, for meeting these demands. OFDM is one of multicarrier communication techniques, whereby data is transmitted in parallel using a large number of subcarriers, and it is known that OFDM has features providing high frequency efficiency and reducing inter-symbol interference under a multipath environment and is effective to improve transmission efficiency.
Studies are being conducted for performing frequency scheduling transmission and frequency diversity transmission using this OFDM on the downlink, when data for a plurality of radio communication mobile station apparatuses (hereinafter simply “mobile stations”) is frequency-domain-multiplexed on a plurality of subcarriers (see Non-Patent Document 1, for example).
In frequency scheduling transmission, a radio communication base station apparatus (hereinafter simply “base station”) adaptively allocates subcarriers for mobile stations, based on the received quality of each frequency band in each mobile station, so that it is possible to obtain a maximum multi-user diversity effect. On the other hand, frequency scheduling is normally performed for each subband, which groups a certain number of neighboring subcarriers into a block, and therefore, not much frequency diversity effect is obtained.
In Non-Patent Document 1, the channel for performing frequency scheduling transmission is referred to as a “localized channel (hereinafter, the “Lch”). The Lch is allocated in subband units or in units of a plurality of consecutive subcarriers. Further, the Lch may be referred to as a “localized resource block (hereinafter, the “L-RB”).”
Non-Patent Document 1 shows an example of dividing one frame (ten milliseconds) into twenty subframes (one subframe=0.5 milliseconds) and including six or seven OFDM symbols in one subframe.
By contrast with this, in frequency diversity transmission, data for mobile stations is allocated to the subcarriers in a distributed manner over the entire band, so that a high frequency diversity effect can be obtained. On the other hand, frequency diversity transmission is performed regardless of received quality for each mobile station, and therefore multi-user diversity effect such as in the frequency scheduling transmission cannot be obtained. In Non-patent Document 1, the channel for performing frequency diversity transmission is referred to as a “distributed channel (hereinafter, the “Dch”). Further, the Dch may be referred to as a “distributed resource block (hereinafter, the “D-RB”).”
Adaptive control including adaptive modulation may be performed for the Lchs and the Dchs on aper subframe basis. For example, to achieve the required error rate, based on received quality information fed back from a mobile station, the base station performs adaptive control for the modulation scheme and coding rate (Modulation and Coding scheme:MCS) of L-ch data and D-ch data.
Upon performing adaptive control, the base station transmits control information on a per subframe basis to the mobile station which is a transmission destination of data in each subframe. Normally, control information is transmitted in SCCHs (Shared Control Channels). Further, control information includes the mobile station ID, RB (Resource Block) numbers, MCS information, and so on. The number of SCCHs in one subframe is the same as the number of mobile stations data is transmitted to in the subframe. Further, control information in an SCCH is transmitted at the beginning of each subframe prior to data transmission. Moreover, transmission power control for an SCCH is carried out on a per mobile station basis. That is, the SCCH for a mobile station located near a cell boundary is controlled to high transmission power, and the SCCH for a mobile station located near a center part of a cell is controlled to low transmission power. By this means, limited power resources are flexibly adjusted between the mobile stations and used effectively.
Non-patent Document 1: R1-050604 “Downlink Channelization and Multiplexing for EUTRA” 3GPP TSG RAN WG1 Ad Hoc on LTE, Sophia Antipolis, France, 20-21 Jun. 2005